The present invention relates generally to a brake actuator, in particular to an air cylinder actuator for air operated brakes of the type typically found on heavy commercial vehicles.
Two types of pneumatic brake actuators are known. Both types of actuators are mounted to a rear of a brake caliper and include a pushrod that interfaces with a corresponding socket on a lever portion of an operating shaft within the brake caliper. Axial movement of the pushrod causes rotation of the operating shaft. The operating shaft, due to its geometry, causes the long stroke, low force input from the pushrod to be converted to a high force, short stroke output to brake tappets and an inboard brake pad.
In one type of actuator, the pushrod is connected to a flexible diaphragm sandwiched between pressed steel sheets to form an air chamber having two sections divided in an air-tight manner by the diaphragm. Pressurized air is introduced into one of the sections, and the pressure differential causes axial movement of the pushrod. The air chamber is attached to the brake caliper via a bolted connection.
The above actuators have a number of drawbacks. For one, the actuator is bulky and thereby constrains vehicle suspension design. Additionally, the space required for the actuator mounting arrangement constrains the brake housing design and limits space inside the housing for internal components. The flexible diaphragm also has a limited life and requires servicing. The pushrod is only held in contact with the brake operating shaft by the brake's internal return spring. In electrical-park applications which act directly on the operating shaft, the pushrod separates from the operating shaft and has to be guided back into position to re-apply the brake when the operating shaft is locked forward for parking. The joint between the brake and the actuator must also be sealed. Finally, the non-pressurized side of the actuator is vented to atmosphere. Further sealing is therefore required between this area and the brake internals to minimize the risk of foreign matter entering the brake and causing corrosion or other damage.
To address some of these problems, a second type of actuator has been proposed that is a piston-type actuator. In such actuators, the flexible diaphragm is replaced by a rigid piston movable in a cylinder of constant cross-section and having some form of sealing arrangement to enable an air pressure differential to be created that causes axial movement of the piston and pushrod. Examples of such actuators are disclosed in, for example, JP60049141 (Hino Motors), EP0944511 (Knorr-Bremse), and WO03/064232 (Volvo Lastvagnar).
However, it has been found that such known designs still suffer from a number of disadvantages, including, for example, the need to have a deep piston to avoid the piston locking or jamming in use. There have also been difficulties in maintaining an effective seal in conjunction with the combined pivoting and axial movement the pushrod and the cylinder undergo when engaged with a operating shaft lever during actuation, which typically have required the pushrod to pivot with respect to the piston and ensuring that the brake caliper remains free from external contaminants.
The present invention seeks to overcome, or at least mitigate, the problems of the prior art.